VLBA Observations Put New Twist on Quasar Jet Model

When a pair of researchers aimed the National Science Foundation's
Very Long Baseline Array (VLBA) radio telescope toward a famous
quasar, they sought evidence to support a popular theory for why
the superfast jets of particles streaming from quasars are
confined to narrow streams. Instead, they got a surprise that
"may send the theorists back to the drawing boards," according
to one of the astronomers.

Quasars are generally thought to be supermassive
black holes
at the cores of galaxies, the black hole surrounded by a spinning
disk of material being drawn inexorably into the black hole's
gravitational maw. Through processes still not well understood,
powerful
jets of particles are propelled outward at speeds nearly
that of light. A popular theoretical model says that magnetic-field
lines in the spinning disk are twisted tightly together and confine
the fast-moving particles into narrow "jets" streaming from the
poles of the disk.

In 1993, Stanford University and Kavli Institute astrophysicist
Roger Blandford suggested that such a twisted magnetic field would
produce a distinct pattern in the alignment, or
polarization, of radio
waves coming from the jets. Zavala and Taylor used the VLBA,
capable of producing the most detailed images of any telescope
in astronomy, to seek evidence of Blandford's predicted pattern
in a well-known quasar called 3C 273.

"We saw exactly what Blandford predicted, supporting the idea
of a twisted magnetic field. However, we also saw another
pattern that is not explained by such a field," Zavala said.

In technical terms, the twisted magnetic field should cause
a steady change, or gradient, in the amount by which the alignment
(polarization) of the radio waves is rotated as one looks across
the width of the jet. That gradient showed up in the VLBA
observations. However, with a twisted magnetic field, the
percentage of the waves that are similarly aligned, or polarized,
should be at its greatest at the center of the jet and decrease
steadily toward the edges. Instead, the observations showed the
percentage of polarization increasing toward the edges.

That means, the astronomers say, there either is something wrong
with the twisted-magnetic-field model or its effects are washed
out by interactions between the jet and the interstellar medium
that it is drilling through. "Either way, the theorists have to
get to work to figure out how this can happen," Zavala said.

When notified of the new results, Blandford said, "these observations
are good enough to warrant further development of the theory."

3C 273 is one of the most famous quasars in astronomy, and was the
first to be recognized as a very distant object in 1963. Caltech
astronomer Maarten Schmidt was working on a brief scientific
article about 3C273 on the afternoon of February 5 that year
when he suddenly recognized a pattern in the object's visible-light
spectrum that allowed an immediate calculation of its distance.
He later wrote that "I was stunned by this development..." Just
minutes later, he said, he met his colleague Jesse Greenstein, who
was studying another quasar, in a hallway. In a matter of another
few minutes, they found that the second one also was quite distant.
3C 273 is about two billion light-years from Earth in the constellation
Virgo, and is visible in moderate-sized amateur telescopes.

The VLBA is a system of ten radio-telescope antennas, each with a dish
25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea
on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands,
the VLBA spans more than 5,000 miles, providing astronomers with the
sharpest vision of any telescope on Earth or in space. Dedicated in
1993, the VLBA has an ability to see fine detail equivalent to being
able to stand in New York and read a newspaper in Los Angeles.

"The extremely sharp radio 'vision' of the VLBA was absolutely
necessary to do this work," Zavala explained. "We used the
highest radio frequencies at which we could detect 3C273's jet
to maximize the detail we could get, and this effort paid off
with great science," he added.